The present invention relates to a switching power supply device that reduces an output voltage setting value when a load is in a standby state.
As this kind of switching power supply device, a switching power supply device of a primary-side control fly back type depicted in FIG. 7 is proposed. The switching power supply device converts single-phase alternating current input to an input terminal 101 into direct current by an AC-DC converting circuit 102, smooths the direct current by a smoothing capacitor 103, and supplies it to a one-end side of a primary winding L11 of a transformer 104. The other-end side of the primary winding L11 is grounded via a switching element 105 and a resistor 106.
An alternating current voltage induced in a secondary winding L2 of the transformer 104 is converted into direct current in a rectifier diode 107, smoothed by an output capacitor 108, and output as an output voltage Vout to an output terminal 109. The output terminal 109 is connected to a load 110.
Additionally, an alternating current voltage corresponding to the secondary winding voltage is induced in an auxiliary winding L12 disposed on the transformer 104. The alternating current voltage is rectified in a rectifier diode 115, smoothed by a smoothing capacitor 116, and supplied to a VCC terminal of a primary-side control unit 117. Furthermore, a voltage across the auxiliary winding L12 is divided in resistors 119 and 120 and supplied to a VS terminal of the primary-side control unit 117.
The primary-side control unit 117 stabilizes the output voltage Vout to a desired output voltage setting value Vref by utilizing the fact that a VS terminal voltage during a period of conduction of the rectifier diode 107 of a secondary side is proportional to the output voltage Vout. Specifically, the primary-side control unit 117 controls a frequency or a duty ratio of a rectangular wave signal PWM output by an error voltage amplifier 117a and a PWM control unit 117b to stabilize the output voltage. The rectangular wave signal PWM is supplied to a drive circuit 118, and the drive circuit 118 performs switching operation of the switching element 105.
The above circuit system is known to be advantageous in that there is no need for an insulation element such as a photocoupler for feedback controlling the output voltage Vout and therefore the number of components can be reduced.
In the circuit system, in order to detect a VS terminal voltage by the primary-side control unit 117, switching operation is regularly performed to conduct the secondary-side rectifier diode 107, thereby monitoring the state of the output voltage Vout. However, while when the switching power supply device is under no load, the output capacitor 108 is charged by the switching operation during the period of conduction of the rectifier diode 107, there is ideally no loss path during a non-conduction period thereof. Due to this, there is a problem in that energy charged in the output capacitor 108 is not discharged and the output voltage Vout continues to increase.
Thus, as depicted in FIG. 7, increase in the output voltage Vout is suppressed by connecting a discharging resistor 121 in parallel to the output capacitor 108. However, a loss (Vout2/Rd) due to a resistance value Rd of the discharging resistor 121 occurs even in standby mode during which no load current flows. Accordingly, from the viewpoint of low standby power, it is desired to maximally increase the resistance value Rd of the discharging resistor 121.
In order to suppress increase in the output voltage Vout even though the resistance value Rd of the discharging resistor 121 is increased, the non-conduction period of the rectifier diode 107 corresponding to a discharging period of the output capacitor 108 needs to be made sufficiently long. In this case, an interval for monitoring the state of the output voltage also becomes long, which causes a problem in that output voltage control is delayed at a time of occurrence of an external disturbance such as input fluctuation or load fluctuation. On the other hand, there is a means for suppressing increase in the output voltage Vout by shortening the conduction period of the rectifier diode 107 by shortening an ON time of the switching element. This means can cause a new problem in that a control terminal voltage of the switching element does not increase up to a threshold value and the switching element is not conducted.
Then, as another means for reducing standby power, a method is employed that reduces the output voltage Vout in standby mode. This means can reduce a loss (Vout2/Rd) of the resistance value Rd of the discharging resistor 121. Furthermore, as depicted in FIG. 7, in the structure in which a power supply voltage Vcc of the primary-side control unit 117 is supplied from the auxiliary winding L12 via the rectifier diode 115, the power supply voltage Vcc is also reduced. Thus, when consumed current of the primary-side control unit 117 is Icc, a loss (Vcc×Icc) of the primary-side control unit 117 can be reduced.
As a conventional technique for reducing the output voltage, for example, a power supply circuit described in JP 2011-97792 A is proposed. In the power supply circuit described in JP 2011-97792 A, output of a secondary rectifier unit connected to a secondary side of a transformer is supplied to a feedback unit, and a switching signal for switching a feedback constant is supplied to the feedback unit from a control unit, thereby changing a resistance value of a voltage dividing resistor of a shunt regulator.
When a load is in an operation state, the power supply circuit controls a voltage that is supplied to the shunt regulator to a low value by using three resistors including two resistors of the secondary rectifier unit side and one resistor of a ground side. On the other hand, when in a standby state or a sleep state, the power supply circuit controls a voltage that is supplied to the shunt regulator to a high value by bypassing one of the resistors of the secondary rectifier unit side. Then, a switching frequency of a switching element connected to a primary-side winding of the transformer is lowered, and an output voltage output from the secondary rectifier unit of the secondary side of the transformer is lowered.
In addition, JP 2012-235618 A discloses a switching power supply circuit provided with two current paths including a first current path and a second current path, used as current paths for generating a feedback signal of a feedback circuit. In the power supply circuit, a P-ON-H signal that is input to the feedback circuit is turned ON at a P-ON time, and a switching element is driven with current of the second current path, without using current of the first current path. At this time, a low level feedback signal is generated. The feedback signal is supplied to a primary-side control circuit via a photocoupler, whereby a secondary-side output voltage of the transformer is controlled at high level. On the other hand, at a standby time, no P-ON-H signal is input, and current is allowed to flow to both of the first and second current paths, thereby driving the switching element with currents of both current paths. At this time, a high level feedback signal is generated and then supplied to the primary-side control circuit via the photocoupler, whereby the secondary-side output voltage of the transformer is switched to a lower voltage.
Furthermore, a switching power supply device described in JP 2013-46423 A detects a duty of a pulse voltage that occurs on a secondary winding of a transformer by a duty detecting circuit. The power supply device changes a feedback signal that is supplied to a primary-side control circuit via a photocoupler when the duty of a pulse signal becomes below a previously determined duty, thereby reducing a secondary-side output voltage of the transformer.